The cutting insert used for such machining operations generally includes an end cutting edge and side cutting edges as viewed from a rake face side. The end cutting edge is straight, and the side cutting edges extend rearward from opposite ends of the end cutting edge. When turning is used for grooving and cutting-off a workpiece, the workpiece is fixed to a main spindle (chuck) of a lathe, and a cutting tool (turning tool) with a cutting insert suitable for the intended machining operation fixed to an end of a holder is attached to a tool post. Then the tool post is moved (longitudinally fed) toward the rotating workpiece at an appropriate feed rate (mm/rev) in the radial direction of the workpiece (a direction perpendicular to the rotation axis).
Chips discharged during grooving and cutting-off flow (are discharged) basically rearward (in a direction away from the rotation axis) because the end cutting edge is generally parallel to or nearly parallel to the rotation axis of the workpiece. If, during machining, a long chip flows rearward or a chip forms a coil shape, the chip may swing laterally and cause scratches on the workpiece, or the chip wraps around the cutting edge or the workpiece. To prevent these problems, a wall (breaker wall) rising obliquely rearward from the rake face is generally provided in a cutting insert of the above type so that chips flowing rearward curl into a flat spiral spring shape. This rake face shape is designed such that chips curl into a flat spiral spring shape. Chips formed during grooving and cutting-off are deformed by shearing resistance and heat, and therefore the discharged chips have a thickness larger than the cutting depth and also have a stripe shape with a width larger than the width of the end cutting edge. In this case, the opposite ends (edges) of the chips rub against the wall surfaces of the machined groove (the groove wall surfaces) or the end surfaces of the cut-off workpieces (the cut end surfaces. These cut end surfaces and the groove wall surfaces are collectively referred to as cut end surfaces), so that the cut end surfaces are scratched and deterioration in surface accuracy (the roughness of the machined surface) occurs. As described above, also during grooving and cutting-off as well as during other turning operations, so-called chip disposability, i.e., how to smoothly dispose of discharged chips while preventing the chips from coming into contact with the workpiece, is important.
In view of the above, various cutting inserts in which the shapes of the end cutting edge and the rake face, etc. are improved have been proposed in order to improve the chip disposability (Patent Documents 1 and 2). Specifically, for example, a recessed groove (a dent) extending in a forward-rearward direction is formed in a central portion of the rake face that includes the end cutting edge and is interposed between the opposite side cutting edges. When this cutting insert with the recessed groove formed therein is used for cutting, the chip is bent such that the widthwise central portion of the chip in its transverse cross section is bent so as to conform to the recessed groove. When the chip flows rearward, portions of the chip close to its opposite lateral edges are lifted by the opposite groove edges (ridge portions) of the recessed groove, and the chip is thereby deformed so as to curl in its width direction, so that the width of the chip is reduced. In this manner, the occurrence of scratches caused by chips rubbing against the cut end surfaces is reduced. Since the wall rising obliquely rearward is provided on the rake face, the chip flowing along the wall surface curls into a flat spiral spring shape and is cut into an appropriate curl length.